Downloaded from genesdev.cshlp.org on September 25, 2021 - Published by Cold Spring Harbor Laboratory Press REVIEW The Hippo pathway: regulators and regulations Fa-Xing Yu and Kun-Liang Guan1 Department of Pharmacology, Moores Cancer Center, University of California at San Diego, La Jolla, California 92093, USA Control of cell number is crucial in animal development a key node to coordinate these cellular processes (Fig. 1). and tissue homeostasis, and its dysregulation may result The Hippo pathway was first defined in Drosophila by in tumor formation or organ degeneration. The Hippo genetic mosaic screens for tumor suppressor genes. Ge- pathway in both Drosophila and mammals regulates cell netic inactivation of genes, including Warts (Wts) (Justice number by modulating cell proliferation, cell death, and et al. 1995; Xu et al. 1995), Hippo (Hpo) (Harvey et al. 2003; cell differentiation. Recently, numerous upstream com- Jia et al. 2003; Pantalacci et al. 2003; Udan et al. 2003; Wu ponents involved in the Hippo pathway have been iden- et al. 2003), Salvador (Sav;alsoknownasShar-Pei)(Kango- tified, such as cell polarity, mechanotransduction, and Singh et al. 2002; Tapon et al. 2002), and Mats (Lai et al. G-protein-coupled receptor (GPCR) signaling. Actin cyto- 2005), all resulted in a similar phenotype with robust skeleton or cellular tension appears to be the master tissue overgrowth. Yorkie (Yki) is the major downstream mediator that integrates and transmits upstream sig- effector of the Hippo pathway (Huang et al. 2005), which nals to the core Hippo signaling cascade. Here, we review regulates a transcription program by interacting with the regulatory mechanisms of the Hippo pathway and discuss transcription factor Scalloped (Sd) (Fig. 2; Goulev et al. potential implications involved in different physiological 2008; Wu et al. 2008; Zhang et al. 2008; Zhao et al. 2008). and pathological conditions. The Hippo pathway is highly conserved in mammals: MST1/2 (Hpo orthologs), Sav1, Lats1/2 (Wts orthologs), Cell proliferation, death, and differentiation are funda- and Mob1 (MOBKL1A and MOBKL1B, Mats orthologs) mental biological processes. Coordination of these pro- form a kinase cascade that phosphorylates and inhibits cesses is critical for a wide range of physiological and YAP/TAZ (Yki orthologs). YAP/TAZ in conjunction with pathological conditions (Pellettieri and Sanchez Alvarado TEAD1–4 (Sd orthologs) mediate major physiological 2007; Galliot and Ghila 2010). During development, an functions of the Hippo pathway (Fig. 2; for reviews, see increase in cell number is required to boost organ and Pan 2010; Zhao et al. 2010a). The nomenclature of many body size; meanwhile, proper differentiation of multiple components of the Hippo pathway in Drosophila and cell types will assure the appropriate function of devel- mammals is different, and a summary of these compo- oped organs. In adulthood, most tissues undergo contin- nents is shown in Table 1. uous cell turnover to maintain functionality. Aged or The core Hippo pathway has been well established in damaged cells are programmed to cell death, whereas both Drosophila and mammals; however, the regulatory adult stem cells may divide and differentiate to replace mechanisms for this signaling pathway are less under- those dysfunctional cells. Under pathological conditions, stood. Recently, by using both genetic and biochemical such as wound healing and organ regeneration, cell approaches, many additional components have been iden- division and differentiation of tissue-specific progenitor tified to modulate the core Hippo pathway (Table 1). In cells will be up-regulated to compensate for the lost cells. this review, we briefly describe the components of the On the other hand, uncontrolled cell proliferation and Hippo pathway and summarize recent advances with decreased cell death lead to hyperplasia or tumorigenesis. respect to Hippo pathway regulation. In addition, we also Detailed mechanisms underlying cell proliferation, cell discuss the implications of Hippo pathway regulation death, and cell differentiation have been extensively in different physiological and pathological conditions. studied; however, how these processes are coordinated The mammalian Hippo pathway is the main focus, al- and integrated is poorly understood. though some Drosophila works are also covered. For a Recently, the Hippo pathway has been shown to pro- detailed review on the Drosophila Hippo pathway, please mote cell death and differentiation and inhibit cell pro- refer to Staley and Irvine (2012). liferation; therefore, the Hippo pathway may function as Core Hippo pathway: a kinase cascade [Keywords: actin; GPCR; Hippo; YAP; mechanotransduction; polarity] MST1/2 are STE20 family protein kinases and can phos- 1Corresponding author E-mail [email protected] phorylate Sav1, Lats1/2, and Mob1 (Wu et al. 2003; Chan Article is online at http://www.genesdev.org/cgi/doi/10.1101/gad.210773.112. et al. 2005; Callus et al. 2006; Praskova et al. 2008). The GENES & DEVELOPMENT 27:355–371 Ó 2013 by Cold Spring Harbor Laboratory Press ISSN 0890-9369/13; www.genesdev.org 355 Downloaded from genesdev.cshlp.org on September 25, 2021 - Published by Cold Spring Harbor Laboratory Press Yu and Guan 2007; Lei et al. 2008; Oh and Irvine 2008), in which the interaction may be mediated by PPxY motifs on Lats1/2 and WW domains on YAP/TAZ (Hao et al. 2008; Oka et al. 2008). Lats1/2 are AGC family kinases and recognize the substrate consensus sequence HXRXXS (Zhao et al. 2007). All five HXRXXS sites on YAP are directly phos- phorylated by Lats1/2 (Zhao et al. 2010b). The phosphor- ylated form of YAP is sequestered in the cytoplasm via a 14-3-3 interaction, resulting in inhibition of target gene transcription (Zhao et al. 2007). Also, TAZ and Yki are phosphorylated by Lats1/2 or Wts, respectively, on mul- tiple HXRXXS sites (Kanai et al. 2000; Dong et al. 2007; Lei et al. 2008; Oh and Irvine 2008; Ren et al. 2010b). In contrast, when upstream kinases are inactive, Yki/ YAP/TAZ will be hypophosphorylated and translocate into the nucleus to exert their functions on gene expres- Figure 1. Implications of the Hippo pathway in cell biology. sion (Kanai et al. 2000; Dong et al. 2007; Zhao et al. 2007; The Hippo pathway modulates cell proliferation, differentiation, Lei et al. 2008; Oh and Irvine 2008; Ren et al. 2010b). The growth, and death. The coordination of different cellular pro- phosphorylation status of YAP/TAZ also regulates their cesses by the Hippo pathway may contribute to diverse physio- protein stability. Phosphorylation of YAP (S381) and logical and pathological conditions such as development, tissue TAZ (S311) by Lats1/2 primes subsequent phosphoryla- homeostasis, and tumorigenesis. tion events by casein kinase 1 (CK1d/e); this sequential phosphorylation results in recruitment of b-transducin repeat-containing proteins (b-TRCP; a subunit of the SCF kinase activity of MST1/2 is enhanced through interaction ubiquitin E3 ligase) and consequently leads to degradation with Sav1, which is mediated by SARAH (Sav/Rassf/Hpo) of YAP/TAZ (CY Liu et al. 2010; Zhao et al. 2010b). domains present in both MST1/2 and Sav1 (Callus et al. Therefore, by affecting YAP/TAZ protein localization 2006). In addition, the thousand-and-one (TAO) amino and stability, phosphorylation by upstream kinases rep- acids kinase or TAOK1–3 has been shown to directly resents a central regulatory mechanism for YAP/TAZ phosphorylate and activate Hpo or MST1/2 (Boggiano (Fig. 2). et al. 2011; Poon et al. 2011). In Drosophila, RASSF YAP/TAZ do not contain intrinsic DNA-binding do- competes with Sav for Hpo and recruits a PP2A com- mains but instead bind to the promoters of target genes plex (dSTRIPAK) to dephosphorylate and inactivate Hpo (Polesello et al. 2006; Ribeiro et al. 2010). However, mul- tiple RASSF isoforms in mammals showed different roles on the Hippo pathway (Praskova et al. 2004; Ikeda et al. 2009), suggesting a divergent role through evolution. MST1/2 directly phosphorylate Lats1/2 at the hydro- phobic motif (Lats1 T1079 and Lats2 T1041), and this phosphorylation is required for Lats1/2 activation (Chan et al. 2005). Mob1, when phosphorylated by MST1/2, binds to the autoinhibitory motif in Lats1/2, which in turn leads to the phosphorylation of the Lats activation loop (Lats1 S909 and Lats2 S872) and thereby an increase of their kinase activity (Chan et al. 2005; Praskova et al. 2008). Sav1 may function as a bridge to bring MST1/2 and Lats1/2 together (Tapon et al. 2002; Callus et al. 2006) and may enhance or inhibit the activity of Lats1/2 upon phosphorylation by MST1/2 or salt-inducible kinases, respectively (Callus et al. 2006; Wehr et al. 2012). The requirement for MST1/2 to activate Lats1/2 might be cell type-dependent. For instance, MST1/2 knockout in mouse livers does not significantly affect Lats1/2 phosphoryla- tion (Zhou et al. 2009), suggesting that additional ki- Figure 2. The core Hippo pathway. MST1/2 phosphorylates Sav, Lats1/2, and Mob; Lats1/2 phosphorylates YAP/TAZ; and phos- nases may regulate Lats1/2 activity. In addition to protein phorylated YAP/TAZ interacts with 14-3-3 and results in cyto- phosphorylation, the protein levels of Lats1/2 kinases are plasmic retention. Moreover, YAP/TAZ phosphorylation leads controlled by Itch E3 ubiquitin ligase-mediated degrada- to protein degradation. When dephosphorylated, YAP/TAZ enter tion (Ho et al. 2011). nuclei and induce gene transcription by interacting with tran- Lats1/2 directly interact